Pitch synchronous reduction of signal envelope

ABSTRACT

BY REDUCING THE ENVELOPE OF A SIGNAL, SUCH AS A SPEECH SIGNAL, IN A NONLINEAR FASHION A MODIFIED SIGNAL IS PRODUCED WHICH, IN ADDITION TO BEING HIGHLY INTELLIGIBLE, EXHIBITS DESIRABLE PROPERTIES. PITCH SYNCHRONOUS GAPS, SMOOTHLY ESTABLISHED IN A REDUCED ENVELOPE SIGNAL IN PLACE OF INTERVALS OF LOW SIGNAL LEVELS, ARE EFFECTIVE IN ELIMINATING NOISE WHICH MASKS LOW LEVEL SIGNALS, REDUCING REVERBERATION, AND IN ADAPTING THE SIGNAL FOR MULTIPLEX TRANSMISSION. ENVELOPE REDUCTION IS EFFICIENTLY ACHIEVED BY CONTROLLABLY CENTER CLIPPING A SELECTED MOFULATION SIDEBAND OF A SIGNAL, FILTERING THE RESULTANT, AND RESTORING THE SIGNAL TO BASEBAND.

"United States Patent US. Cl. 328--169 Claims ABSTRACT OF THE DISCLOSUREBy reducing the envelope of a signal, such as a speech signal, in anonlinear fashion a modified signal is produced which, in addition tobeing highly intelligible, exhibits desirable properties. Pitchsynchronous gaps, smoothly established in a reduced envelope signal inplace of intervals of low signal levels, are effective in eliminatingnoise which masks low level signals, reducing reverberation, and inadapting the signal for multiplex transmission. Envelope reduction isefficiently achieved by controllably center clipping a selectedmodulation sideband of a signal, filtering the resultant, and restoringthe signal to baseband.

BACKGROUND OF THE INVENTION This invention relates generally to theprocessing of signals and in particular to the modification of speechsignals such that the envelope of a signal is reduced in a nonlinearfashion.

(1) Field of the invention Much effort has been expended to developtechniques for the modification of a signal, such as a speech signal, toplace it in a form suitable for more eeconomic transmission to a distantstation without, however, seriously affecting its intelligibility. Onetechnique that has found favor involves so-called envelope reduction ofthe signal. By continually reducing the envelope of a signal at a pitchsynchronous rate, gaps are created in the signal at irregular intervals.The gaps are effective to eliminate noise components that oveerride weaksignal portions, and may be used for time sharing a transmission channelwith other signals. In essence, envelope reduction of speech constitutesone technique for increasing communication efiiciency.

(2) Description of the prior art Interrupted speech signals aregenerally prepared by interrupting a signal, as by chopping, whenever itis determined that the signal is either contaminated by spuriouscomponents or that it is conveying insufficient information to warrantretention. For example, it is conventional in receiving signals subjectto noise contamination to detect periods of severe noise interferenceand chop the signal to remove these components. The informationbearingsignal is, of course, lost in the process. For multiplex transmission itis customary to seek intervals of low signals level, preferablysynchronous with the signal content, and to delete signal portionsduring such intervals to permit other signals to be transmitted in theirstead.

If the required gaps in the signals are created simply by interruptingthe signal instead of waiting for a period of silence, severe degrationof signal quality and a loss of intelligibility results. The primaryreason for speech degradation as a result of signal interruption orchopping is twofold: (1) the interruptions introduce discontinuities inthe signals, two for each interruption, and (2) the interruptions,unless they fortuitously occur at pitchsynchronous intervals, create aninharmonic signal. Even 3,566,285 Patented Feb. 23, 1971 if the timingintervals are judiciously selected, for example, by seeking times of lowsignal amplitude, a reproduction of the inharmonic signal is distorted.Moreover, rather complex signal analyzing equipment is required to seekout appropriate chopping times with only a slight chance of improvementover purely random chopping. Obviously, the extent of the gaps carvedout of a speech signal and the manner in which they are created play animportant role in determining the naturalness of a reconstituted speechsignal.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto improve the manner of interrupting a relatively continuous signal.

It is another object of the invention to reduce the envelope of a signalin a nonlinear fashion such that the signal is caused to maintainrelatively full amplitude during periods of strength but to fallsmoothly to zero during periods of weakness.

The outlined difiiculties are overcome in accordance with the presentinvention by reducing the envelope of a continuous speech signal suchthat gaps are smoothly created in the signal at a pitch synchonous rate.Envelope reduction is effectively achieved, with a minimum ofprocessing, by modulating an applied signal to shift it to a higherfrequency, center clipping a selected side frequency of the modulatedsignal, e.g., one sideband, selectively filtering the center-clippedsignal, and restoring the signal to its original frequency range.

Processing of this sort permits the signal to be reconstituted withselected frequency components only retained; spurious componentsinharmonically related to the fundamental are removed. The reconstitutedsignal is thus selectively distorted to exhibit a nonlinear input-outputcharacteristic such that low level signals are effectively removed,while higher amplitude signal levels are preserved. Transitions fromintervals of full signal strength to those of zero signal strength aresmooth, i.e., there are no servere discontinuities. No actual choppingtakes place so that the usual switching noise is avoided. Because of thesmooth switching transitions, however, all of the advantages of aninterrupted signal are achieved without accompanying degradation of thereconstituted signal. Speech reproduced with such processing is fullyintelligible, of high quality, and devoid of contaminating components,such 'as noise overriding weak signal components. The invention is thuswell suited to the processing of a signal to prepare it for multiplextransmission with other signals.

Signal amplifying and processing equipment, such as public address soundsystems, hands-free telephone systems, and the like, also benefit fromsuch signal processing. A reduced envelop signal, prepared in accordancewith the invention, serves to reduce noise in speech signals and,importantly, the reduction takes place when noise is perceptibly mostdeleterious, i.e., during silent intervals between connected utterancesand, more generally, during instants of low speech envelope. Moreover,reverberation effects are curtailed and system stability is improved.

DESCRIPTION OF THE DRAWINGS The invention will be fully apprehended fromthe following detailed description of an illustrative embodiment thereofread in connection with the appended drawings in which:

FIG. 1 is a block schematic diagram of a signal processing networkconstructed in accordance with the invention; and

FIG. 2 is a set of curves illustrating signal level discriminationcharacteristics typical of those achieved by the center clippingapparatus of FIG. 1.

9 DETAILED DESCRIPTION A signal processing network for developingenvelope reduced signals is illustrated in FIG. 1. Continuous inputsignals applied to terminal 10 of the system are delivered to balancedmodulator 11 in order to elevate the signals to the frequency of carriersignals supplied by oscillator 12. In well-known fashion,sum-and-difference signals are produced by the modulation operation.Bandpass filter 13 serves to eliminate all but a selected one of theresulting modulation products. Consequently a side frequency of theinput signal appears at the output of filter 13. This signal is suppliedto center clipping network 14 which is momentarily adjusted to clipapplied signals severely in order to preserve only the peak waveintervals.

Center clipping a signal produces a new wave rich in higher orderharmonics. Consequently, by passing the center clipped wave through asuitable filter with a selected passband, e.g., bandpass filter 15, theadded higher order harmonics in the clipped signal are removed. Filter15 may, in practice, have a passband substantially identical to that offilter 13. The signal developed at the output of filter 15 thus containsin-band components that were present in the signal delivered by filter13, but none of the out-of-band harmonics, or harmonic productsresulting from the center clipping operation. Periodicity of the inputwave is, however, preserved.

The resulting wave is then demodulated in network 16, under control ofwaves from oscillator 12, to produce at output terminal a basebandsignal corresponding to the input signal but characterized by a reducedenvelope. Obviously, independent signal sources may be used in place ofsource 12 to energize modulators 11 and 16. If desired, the basebandsignal may be additionally filtered to tailor its characteristic to adesired form.

In order to accommodate a wide range of input signals and in order tomaintain relatively constant envelope reduction for such signals, it isdesirable continuously to adjust the degree of center clipping employedin the envelope reducing system. Any one of a variety of techniques maybe employed. One suitable implementation is shown in FIG. 1. The bandlimited input signal produced by filter 13 is supplied to envelopedetector 17, of any desired construction, to produce a control signalwhose variations are proportional to signal envelope. The signal isfurther smoothed by low pass filter 18 and used to control adjustablegain amplifier 19. Amplifier 19, which may be any suitable unit whosegain is responsive to applied control signals, produces a slowly varyingcontrol signal whose magnitude is proportional to the magnitude of theenvelope of the input signal. As a refinement, envelope detector 17 andfilter 18 may be equipped with adjustable holdover circuits so thatdilferent time constants may be established to assure appropriate attackand release times for the control signal used to set the center clipper.Tl1is has been found to be helpful with certain classes of signals. Thecontrol signal developed by amplifier 19 is suitable for continuouslycontrolling the clipping level.

Center clipping systems suitable for use in the practice of theinvention may take any one of a number of forms well known in the art.One satisfactory form is shown within the dashed outline of unit 14. Itincludes a peak limiting network 21 and a combining network 22. Signalsfrom filter 13 are supplied directly to combining network 22 and to peaklimiter 21. Peak limited signals thereupon are subtracted from thefiltered input signal to produce a signal containing only the peakextremities of the input signal, i.e., a center clipped signal.

Regardless of the form of center clipping employed, the effect onapplied signals is much the same; only signals which exceed a fixedthreshold level are retained in the output. This characteristic isillustrated in FIG. 2. For an ideal center clipper, a sharp break occursat a selected threshold; there is no output below this threshold, butthere is a reduced output, following a linear inputoutputcharacteristic, above the threshold. An asymmetric power law deviceapproaches this characteristic. A modi fied square law device may beused, and a cube law system yields good results, as depicted in FIG. 2.As indicated above, the threshold level, (for both positive and negativesignal excursions) is, in accordance with the invention, adjustedautomatically in response to a measure of the magnitude of the inputsignal envelope, by means of a control signal from amplifier 19, tomaintain an essentially constant relative clipping level.

Reduced envelope signals delivered, in accordance with the invention, atoutput terminal 20 (FIG. 1), characteristically preserve high levelenvelope components but eliminate low level components. Thus, duringperiods of low input, an output signal is produced which ischaracterized by a gap, i.e., by zero signal level below a selectedinput level threshold. Moreover, transitions from intervals ofabove-threshold level signals and below level signal gaps are smooth andoccur at a pitch synchronous rate. Therefore, no switching transientsare interposed in the output signal, and signal discontinuity eflfectsare minimized. The regenerated envelope thus exhibits a nonlinearinput-output characteristic such that low level sig nals are effectivelyremoved while higher amplitude signals are preserved.

Use of a network with such a characteristic has been found extremelyuseful in systems in which noise com ponents are comparable in magnitudeto weak signal level portions. By smoothly reducing the envelope of thesignal to remove low level signals, which are masked by noise anyway, asignal is preserved which contains only portions whose levels aresufficiently great to withstand the influence of noise.

Similarly, reverberation in speech signals may be advantageously removedby envelope reduction. The action is particularly effective because ofthe smooth signal level control available. With a fast time constant,loud vowel sounds are noticeably limited; yet following sounds are notclipped.

In acoustic systems, such as public address sound systems, hands-freetelephone systems, and the like, it has been found that acousticfeedback, which results in socalled sing-around characterized by ahowling sound, is effectively reduced since frequent gaps are created inthe signal to break up sustained oscillations. The nonlinearcharacteristic of the envelope reducing apparatus thus appreciablyimproves the stability of systems prone to acoustic'feedback since thestability of such systems depends on gain averaged over short timeintervals rather than on the instantaneous gain of the system.

The invention has been described, by way of example, in terms of aparticular form of apparatus. It is to be understood, however, that thedescribed arrangement is merely illustrative of the application of theprinciples of the invention. Numerous other arrangements may be devisedby those skilled in the art without departing from the spirit and scopeof the invention.

What is claimed is:

1. Signal processing apparatus, which comprises, in combination,

means for selecting for processing a modulation sideband of an appliedbaseband signal,

means for controllably center clipping said selected modulation sidebandsignal, and

means for restoring said center clipped signal to baseband.

2. Apparatus for reducing the envelope of a signal which comprises, incombination,

means for shifting an applied signal to a higher frequency range,

means for center clipping a selected side frequency of said shiftedsignal, and

means for selectively restoring said center clipped signal to itsoriginal frequency range.

3. Apparatus for reducing the envelope of a signal which comprises, incombination,

means for modulating an applied signal to shift it to a higher frequencyrange,

means for center clipping a selected sideband of said modulated signal,

means for selectively filtering said center clipped signal,

and

means for demodulating said filtered signal to shift it to the originalfrequency range of said applied signal.

4. Apparatus as defined in claim 3 in further combination with means forcontrolling the extent of center clipping in response to variations in aselected characteristic of said applied signal.

5. Apparatus as defined in claim 3 wherein, said means for centerclipping a selected sideband signal includes,

means supplied with a selected sideband signal for selectively limitingthe peak excursions thereof, and means for subtracting said peak limitedsignal from said sideband signal.

6. Apparatus as defined in claim 5 wherein the extent of limiting bysaid limiting means is controlled by variations in the envelope of saidselected sideband signal.

7. Apparatus for reducing the envelope of a signal, which comprises, incombination,

means for developing a relatively high frequency signal with an enveloperepresentative of an applied communication signal,

means for adjustably center clipping said envelope signal,

means for developing an analog signal representative of said envelopesubstantially devoid of out-of-band harmonic signals,

means for developing a baseband signal representative of said analogsignal representation, and

means for continuously adjusting the degree of said center clipping inaccordance with variations in said applied communication signal, 8.Apparatus for reducing the envelope of a signal which comprises, incombination,

means supplied with a signal for modulating said signal with a selectedcarrier signal, means for eliminating all but a selected one of theresulting modulation products, means for center clipping said selectedmodulation product to preserve only the peak. intervals thereof, filtermeans with a selected passband for removing higher order harmonicsassociated with said center clipped signal, and means for producing abaseband signal from said filtered signal. 9. Apparatus as defined inclaim 8 wherein said means for center clipping said sideband signalexhibits a cube law characteristic. 1

10. Apparatus as defined in claim 8 wherein said means for centerclipping said sideband signal exhibits an asymmetric power lawcharacteristic.

References Cited UNITED STATES PATENTS 1,550,660 8/1925 Affel 325-6'5X3,028,554 4/1962 Hilliard, Jr. 328l17 3,094,667 6/1963 Brown 328-162DONALD D. FORRER, Primary Examiner J. ZAZWORSKY, Assistant Examiner US.Cl. X.R.

